Dual frequency reception is of particular relevance in receivers of satellite-based positioning systems such as the Global Positioning System (GPS) and the forthcoming Galileo system.
The GPS system uses 24 satellites, distributed over 6 orbital planes 20.200 km above the Earth. The satellites take 12 sidereal hours to orbit the earth.
All GPS satellites emit signals on 2 frequencies: L1 (1575.42 MHz) and L2 (1227.6 MHz). Three pseudo random noise (PRN) ranging codes are currently in use:                The coarse/acquisition (C/A) code modulates the L1 carrier, which carries the navigation message, and has a 1.023 MHz chip rate and a period of 1 ms        The precision (P) code modulates L1 and L2 carriers (P1, P2) and has a 10.23 MHz rate and a period of 7 days        The Y-code is presently used in place of the P-code (the Y-code is obtained by coding the P-code with a W-code that is only known to authorized users) because anti-spoofing (AS) is activated since 1994. The corresponding observables are Y1 and Y2.        
In the scope of the modernization of the GPS, a new L2 Civil signal (L2C) will be transmitted by modernized IIR (IIR-M) and all subsequent GPS satellites. Then both the L1 and the new L2C signal will be available to civil (non-authorized users). Furthermore, a third civil signal L5 will be transmitted on a third carrier at 1176.5 MHz.
GPS receivers can be hand-carried or installed e.g. on aircraft, ships, tanks, submarines, cars, and trucks. These receivers detect, decode, and process GPS satellite signals. They must lock onto the signals from four of the GPS satellites to give a full three-dimensional position.
Different types of receivers make use of different parts of the GPS signal structure. The basic observable at a GPS receiver is currently the C/A-code on the L1 carrier. The signals allow the GPS receiver to calculate the distances to four satellites, and with that data, it can calculate a stand-alone position on the Earth's surface in latitude and longitude within +/−100 meters 95% of the time. Since the four signals received are stabilized by atomic clocks, the timing accuracy of an ordinary digital clock is sufficient for the calculations done in the GPS receiver unit. This is achieved by exploiting the fact that only three satellite signals are actually required to determine the position of the GPS receiver, and by using the additional degree of freedom represented by the fourth received satellite signal to correct for timing errors induced by the low-quality digital clock used by the GPS receiver.
When the GPS receiver receives a PRN code from a satellite, it generates a plurality of replica codes and correlates the received PRN code with the plurality of replica codes. The replica codes that achieves the highest correlation identifies the transmitting satellite, and the time interval by which this replica code must be shifted in the receiver to maintain maximum correlation with the received PRN code, multiplied by the speed of light, is approximately equal to the satellite range. The corresponding position of the satellite can be extracted from an almanac that is implemented into the GPS receiver. The satellite range is called the pseudo range because the measurement must be corrected by a variety of factors to obtain the true range. The corrections that must be applied include signal propagation delays caused by the ionosphere and the troposphere, the satellite clock error, and the GPS receiver clock error. The true geometric distance to each satellite is obtained by applying these corrections to the measured pseudo range.
By using dual frequency phase measurements, and knowledge of the inverse square relationship between the group delay and the frequency of each carrier (for instance, L1 and L2), a simple linear correction of the delay induced by the ionosphere can be derived. This correction can be expressed as follows:τL1=1.5336·ΔτL1,L2 where τL1 is the ionospheric delay on carrier L1 and ΔτL1,L2 is the difference in delay between L1 and L2.
In order to increase the accuracy of satellite-based positioning, a GPS receiver thus has to receive both the L1 and the L2 carrier (or other combinations of L1, L2 and L5 in the future, as for instance L2 and L5), requiring two complete receiver structures to be implemented into the GPS receiver, and thus increasing both the size and costs of the GPS receiver.
To alleviate this problem, U.S. Pat. No. 6,675,003 B1 proposes a mixer structure that is capable of frequency converting both the L1 and the L2 signals. To this end, the received L1 signal is frequency-converted to a first intermediate frequency, and the received L2 signal is frequency converted to a second Intermediate Frequency (IF). The obtained IF signals are then simultaneously frequency-converted to final IF with a single Image Reject (IR) mixer that uses a mixer frequency that is approximately between said first and second IFs. This IR mixer can either produce the L1 signal converted to said final IF at a first output and the L2 signal frequency-converted to said final IF at a second output, or switch between the L1 signal converted to said final IF and said L2 signal frequency-converted to said final IF, wherein this switched signal is output via a single output of said IR mixer.
However, the mixer structure proposed in U.S. Pat. No. 6,675,003 B1 still requires two mixers for the Radio Frequency (RF) to IF conversion and, to accomplish the switching, furthermore reveals a rather complicated architecture.